The vapor-liquid equilibria (VLE) of the nitric acid system is complicated due to the disassociation of the compounds in the liquid into ions. Various models have been used, including the electrolyte version of NRTL, Henry's Law with disassociation in the liquid, and Pitzer methods. The Pitzer method appears to be the most accurate for high acid molality . The version of Prode Properties that I am using doesn't include the Pitzer method so I am starting with simplistic approaches to get the model framework "up and running". Eventually I will obtain the Prode extended version and convert to the Pitzer model.
The results reported in the blog to this point have used the SRK model for the vapor fugacity and NRTL for the liquid fugacity. The only binary interaction parameters used have been for the H2O-HNO3 pair. Disassociation of HNO3 has not been addressed.
With this simple approach, I am getting the expected acid concentration for the pre-condenser liquid, i.e. about 40 wt%. However, Carberry  also compiles the VLE data from four references and plots the results in his Figure 6-18. The variables plotted are a ratio of gas partial pressures, K4 (see below), and the weight percent HNO3 in the liquid. He also fits the data to the formula below.
The results from this initial model were not good. I obtained values of K4 several orders of magnitude below the data.
Next, I tried using Henry's Law (HL) for all of the gases, O2, N2, NO, NO2, N2O4 and also from subsets of that list. The HL parameters were obtained from a wonderful compilation .
Results improved but the improvement was mainly in the low acid concentration region. The Henry's Law approach has been used by others, but they also had a liquid phase model (Pitzer) that accounted for disassociation, ion interaction and the non-ideal liquid mixture. That will probably be the direction to go, eventually.
A direct approximation
As an interim approximation, I have replaced the VLE fugacity constraint for NO with the logK4 constraint. In effect, this new constraint is forcing the vapor concentration of NO to satisfy the logK4 formula. The N2O4 vapor pressure is also included in K4 so I could have chosen it instead of NO. However, since the model was obtaining low K4 values, and since it is likely that the solutes are "salting out" the gases (i.e. increasing the equilibrium partial pressures), it seems more likely that the NO pressure would increase than for the N2O4 pressure to decrease.
The rxtflash solve block modifications are shown below.
In order to remove the NO fugacity constraint, I created a NullI diagonal matrix of 0's and 1's. This matrix is then used as shown above.
Results using the logK4 constraint
As shown below, the model now matches some important data represented by the Carberry formula. Where no red line appears, it has been covered by the blue line.
This modification hasn't changed the acid concentration for the condenser liquid, so it appears that I have a reasonable approximation for the VLE. I can now proceed with the staged absorber model.
 Nichols, T. T., and D. D Taylor. "Thermodynamic Phase And Chemical Equilibrium At 0-110 ° C For The H+-K+-Na+-Cl--H2O System Up to 16 Molal And The HNO3-H2O System Up To 20 Molal Using An Association-Based Pitzer Model Compatible With ASPEN Plus", 2003. Idaho National Eng. and Environmental Laboratory Report INEEL/EXT-03-01167
 Carberry, J. J., Chemical and Catalytic Reaction Engineering, Dover (2001)
 Sander, R. “Compilation of Henry’s Law Constants, Version 3.99.” Atmos. Chem. Phys. Discuss. 14 (2014): 29615–521. doi:10.5194/acp-15-4399-2015.